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Deglaciation of the northwestern Laurentide Ice Sheet in the central Mackenzie Valley opened the northern portion of the deglacial Ice-Free Corridor between the Laurentide and Cordilleran ice sheets and a drainage route to the Arctic Ocean. In addition, ice sheet saddle collapse in this section of the Laurentide Ice Sheet has been implicated as a mechanism for delivering substantial freshwater influx into the Arctic Ocean on centennial timescales. However, there is little empirical data to constrain the deglaciation chronology in the central Mackenzie Valley where the northern slopes of the ice saddle were located. Here, we present 30 new 10 Be cosmogenic nuclide exposure dates across six sites, including two elevation transects, which constrain the timing and rate of thinning and retreat of the Laurentide Ice Sheet in the area. Our new 10 Be dates indicate that the initial deglaciation of the eastern summits of the central Mackenzie Mountains began at ∼15.8 ka (17.1–14.6 ka), ∼1000 years earlier than in previous reconstructions. The main phase of ice saddle collapse occurred between ∼14.9 and 13.6 ka, consistent with numerical modelling simulations, placing this event within the Bølling–Allerød interval (14.6–12.9 ka). Our new dates require a revision of ice margin retreat dynamics, with ice retreating more easterly rather than southward along the Mackenzie Valley. In addition, we quantify a total sea level rise contribution from the Cordilleran–Laurentide ice saddle region of ∼11.2 m between 16 and 13 ka. [ABSTRACT FROM AUTHOR]

The peak of the Late Wisconsinan was the only time the two main North American ice sheetscoalesced in the foothills of the Rocky and Mackenzie mountains of western Canadafrom near the Montana border to the Northwest Territories. Following the LastGlacial Maximum the western margin of the Laurentide Ice Sheet detached from theCordilleran ice masses and a wedge of unglaciated terrain opened. Reconstructingthe opening of the ice free corridor has been of great interest for archaeology andpalaeoecology because it allowed for the migration of species between Beringia andthe regions south of the ice sheets. However, the timing of the corridor openingis still ambiguous. Here we review the available ages that constrain the corridoropening and present new erratic boulder 10Be exposure age datasets situated atthe northern and southern ends of the corridor. The first dataset of 16 new agesstretches along the Rocky Mountain Foothills in Alberta between 49 and 53 ˚ N,and the other covers the southern Franklin Mountains in Northwest Territories (14ages at ∼63 ˚ N). The results indicate a detachment of the western margin of theLaurentide Ice Sheet from the Rocky Mountain Foothills in southern Alberta atabout 15 ka. In the north, the Mackenzie River valley at 63 ˚ N experienced a rapiddrawdown of the ice sheet surface and a retreat of the ice sheet margin at about 14.5 ka,possibly in connection with the Bølling warming. The 10Be ages indicate an almostsimultaneous detachment of the western Laurentide margin from the Cordillera in thenorth and south. This contrasts with the existing ice margin chronology, based on14C data, which proposes that ice sheet separation in the Mackenzie River valleyoccurred at least 2000 years after separation in southern Alberta. In order to betterconstrain the final opening of the ice free corridor, more exposure ages are needed fromnorthern Alberta, north-eastern British Columbia, and southern Northwest Territories. [ABSTRACT FROM AUTHOR]